Movable actuating coil transducer array



July 28, 1959 w. T. HARRIS MovABLE ACTUATINQCOIL TRANSDUQER ARRAY FiledApril 13. 1954 FIG.

INVENTOR w/a/e r Aff/.s

,\ ATTORNEYS Unite Wilbur T. Harris, Southbury, Conn., assignor to TheHarris Transducer Corporation, Woodbury, Conn., a corporation ofConnecticut Application April '13, 1954, Serial No. 422,787

11 Claims. (Cl. .M0-9)V My invention relates to improved electroacoustictransducer means and' in particular to electromagnetic transducers, asof the character described in greater detail in my copending patentapplication, Serial No. 241,470, filed August 11, 1951.

It is an object of the invention to provide an improved transducer ofthe character indicated.

It is another object to provide an improved means for coupling atransducer of the character indicated to the medium into which thetransducer is to radiate or is otherwise to respond.

It is a further object to provide means for vastly improving theradiating eiliciency of transducers of the character indicated.

Another object is to provide a transducer with improved high eliciencyand broadband characteristics.

A further object is to provide an improved transducer utilizing anacoustically hard structure over a substantial part of the radiatingface thereof, whereby the prospect of damage due to mishandling may beminimized.

Other objects and various Ifurther features of novelty and inventionwill be pointed out or will occur to those skilled in the art from areading of the following specification in conjunction with theaccompanying drawings. In said drawings, which show, for illustrativepurposes only, preferred forms of the invention:

Fig. 1 is a fragmentary sectional view of a transducer incorporatingfeatures of the invention;

Fig. 1A is a simplified fragmentary plan view on a reduced scale andpartly broken-away, to illustrate the array configuration in Fig. l; and

Figs. 2 and 3 are views similar to Fig. l, but illustratingmodifications.

Briefly stated, my invention contemplates the creation of a highlyeilicient broadband transducer from an acoustically hard structure,which presents to the radiating medium, such as water, an acousticallyhard, essentially continuous plate; I term this plate a baille becauseof its property to reilect acoustic signals. At sp-acings substantiallyless than a wavelength, preferably less than one-tenth Wavelength at thedesired response frequency, relatively small openings are provided inthe baille, and electrically driven force elements are supported forreciprocation in these openings. In the forms to be described, the forceelements are essentially electrodynamic coils, which, if formed ofsufficiently wide laminated-strip mateal, may themselves effectivelyconstitute the radiating or driving areas. Alternatively, variousarea-transforming piston configurations may be applied to the coils inorder to assure greatest eiciency of force application Within the areasof baille openings.

In one general form to be described, the baflle openings are circularand provided in a two-dimensional array, there being a separateelectrodynamic force coil for such opening. In another general form, theopenings are parallel slots, spaced as indicated with respect to thewavelength, and elongated racetrack-shaped force coils are provided,with opposed stretches in adjacent slots.

States Patent ECC Referring to Figs. 1 and 1A of the drawings, myinvention is shown in application to an electrodynamic transducer, inwhich the baille plate 10 is a separate member of extensive area andprovided with a plurality of openings 11-12-13 therein, said openingsbeing spaced by an amount S, representing something substantially lessthan a wavelength at the desired response frequency; for example, thespacing S may be less than one-tenth of a wavelength. Adjacent thenon-radiating side of the baille 10, I provide for each of the openings11-,12-13 a sep'- arate magnetic-core structure 14-15--16, defining,symmetrically with respect to each of such openings and vadjacent thenon-radiating side of baille 10, an air gap such as the annular air gap17 in the case of magnet 15. The air gap 17 is defined between theenlarged head or pole piece 18 of an upstanding central member secured,as by bolt 19, in the base of a generally cup-shaped magnetic core; thecore 15-18 may be permanently magnetized, but I show that polarizingllux is established in the gap 17 by coil means 20. It will beunderstood that the coils 20 [for each of the magnet structures 14-15-16may be energized in series or in parallel, depending upon theirimpedance with respect to that of the power supply.

Supported for reciprocation in each of the air gaps, as at 17, I provideelectrodynamic-coil means, and for the case shown in section in Fig. 1,the coil 21 is a helical development of laminated conductive stripssuitably consolidated into unit-handling relation and supported on thecore structure 20, as by readily yieldable acoustically absorbent means22 (such as air-filled rubber). In korder to provide area transformationfor eilcient application of electrodynamically generated forces in therelatively restricted area of opening 12 at the radiating face of baille10, I have shown a piston 23 carried by each of the coils, theconnection being through suitable incompressible insulating means 23.The piston 23 is preferably of lightweight construction, as of magnesiumor aluminum, and is excavated as much as tolerable, as suggested in thedrawings. The piston 23 may be in effect a truncated cone, -with thesmaller or truncated area substantially in the plane of the radiatingface of baille 10. Additional locating support for the coil may beprovided by a relatively thin sheet-metal diaphragm 24, peripherallysecured at the opening 12 and centrally secured to the truncated face ofpiston 23. For sealing purposes, and to present a clean appearance, aboot or outer layer 25 of acoustically transparent material may bebonded to the diaphragms 24 and baille 10.

The structure may be completed by employment of heavily ilanged pieces2.6, constituting a housing to which baille 10 is bolted, as at 27. Thebaille 10 may be coextensive with the outwardly extending flanges 28 ofmember 26; but in any case I prefer to avoid loss of efciency due to endeiects, by providing further acoustically hard baille radiating surfacelaterally outside the array of force elements, to a lateral extentsubstantially exceeding the spacing S between force elements.

For pressure-equalizing purposes, so as to render the transducer lessdepth-dependent, the side-ilange pieces 26 may project substantiallyrearwardly of the backing plate 29 for the magnet assemblies, so as tosupport a back housing plate 30 spaced rearwardly of the plate 29 andldefining therebetween a cavity or chamber 31. Within the chamber 31, Ishow a bag 3?. of flexible material having an external opening at 33, soas to be free-ilooding. Air-bleed passages 34 (in the backing plate'29)and 35 (in the magnet structure) provide a means for assuring ambientpressure in the region in which the electrodynamic coil is suspended, aswill be understood.

In use, if the coil elements for the respective force elements aredriven in phase, the described small-spot array radiates like a largediaphragm driven at larger torce but 3 smaller amplitude; to the mediumthis diaphragmappears to have the size of the baille. The forces andamplitudes are subject to a transformation ratio approximating thetotal-to-active arearatio, as long as oneassllmes thespacing Sto be arelatively smallV fraction of a wave.- length. Since the alternatingacoustic pressure cannever be appreciably diierent at two `points whicharepseparated only a very small fraction of a wavelength, theY drivenspots are not likely to produce cavitation effects when driven at highintensity. The impedance transformation greatly improves theelectroacoustic conversion efficiency.

In Fig.'2, I show application of theprinciples of my invention to aparallel-gap array, as of the vtype disclosed in the above-mentionedcopending application. This type of construction comprises essentially amagnetcore structure 40 having elongated parallel ygaps41-42, spaced (asindicated above) by the amount S, representing a small fraction of awavelength. The magnet-COI@ material may itself be at least the centralpart Of the acoustically hard baille structure, so that no additionalbaille need be provided, except at lateral flanges 28 of `the housing26; the air gaps 41-42 are, therefore, strictly in the baille surface.Separate coils 43--441-45 may be excited with polarizing current so asVto assure the desired distribution of polarity at theW respective gapsl1-42, and in each two adjacenty of said gaps I-'may support opposedstretches 46-47 of a single racetrack-shaped coil, which may be ahelical development of laminated conductive strip material. The frontface of such coil 46-47 may be substantially in` the` plane of theradiating face of the baille, that is, the face48 of core structure 40;and I have shown thin, yieldable, metal diaphragms 49 peripherallysecured to the baile face 48 at the gaps, and centrally secured to therespective. stretches of the coil means 46-47. As in the caseof Fig. l,a boot of acoustically transparent sealing material 50 may surround thestructure. Also, when. adapted for Apressure-equalization, apertures, asat 51, may communicate between the space behind the magnet Vcore 40 .andthe region in which coil means 46-47is supported; thepressure-equalizing means may be ofthe typedescribed at 32 in Fig. l andis therefore not shown in Fig. 2.

In Fig. 3, I show a modification of the structure of Fig. 2, in which Iemploy area-transforming piston elements 55 carried by the respectivecoil means 46-47, and preferably of magnesium, aluminum, or otherlightweight material. The function served by the piston ele- 'ments 55is eifectively to enlarge the force-transmitting area; elements 55,therefore, may be of trapezoidal crosss'ection and may project slightlybeyond the bafe surface 48. It is convenient to provide a thin-metalYdiaphragm 56 for support of the coils, as by securing the same both tothe coils and to the unexcited structure in a common plane. For thisreason, I have applied a layer 57 of acoustically transparent materialover the baille area, except at the gaps 4142, and at these gaps thematerial'57 may be ilared outwardly, as shown, to accommodate thesectional contour of piston elements S. The 'structure of Fig. 3 isotherwise as described for Fig. 2, and therefore the same referencenumerals are `fused.

It will be appreciated that I have described relatively `simpletransducer structures lending themselves to improved efliciency andexhibiting broadband characteristics. These structures are furthercharacterized by extreme ruggedness inasmuch as the exposed radiatingsurface is substantially entirely acoustically hard and may therefore bea heavy steel plate.

While I have described the yinvention in detail for the preferred formsshown, it will be understood that modications may be made within thescope ofthe invention as defined in the claims which follow.

I claim:

l. vIn a transducer of the character indicated, a ,passive -bafe plateof acousticallyhard material having two relatively small laterallyspacedopenings in the radiating face thereof and spaced by an amountrepresenting a small fraction of a Wavelength at the desired responsefrequency, flexible diaphragm means peripherally secured at each of saidopenings and covering each of said openings, magnetic-circuit meansestablishing air gaps adjacent the non-radiating side of said baille andsymmetrically disposed with respect to the axes of said openings, andelectrodynamic driving means in said-respective gaps and extending indirect force-transmitting relation with the back side of each of saiddiaphragms.

2. A transducer accor-ding to claim l, in which the lateral extent ofsaid baffle plate outside the space of said openings substantiallyexceeds the space between said openings.

3. A transducer according to claim 2, and including a boot ofacoustically transparent sealing material overstanding the radiatingsurface of said baffle plate and of said diaphragms.

4. In a transducer of the character' indicated, magneticcore meansdening a plurality of spaced elongated gaps, a movable strip ofconductive material Vincluding `opposed stretches in the respective eldsof two `of said gaps, said magnetic-core means being formed with anessentially continuous surface constituting an accustically hard batheinterrupted only at said gaps, whereby said movable conductive strip mayin part ill said gaps to provide a substantially `continuous radiatingsurface, and means ilexibly supporting said strips in said gaps forreciprocaton on axes generally normal to said surface.

5. A transducer laccording to claim 4, in which said movable strip `ofconductive material comprises a lhelical ycoil containing a plurality ofturns of laminated strip material, there being a Vplurality oflaminations in the field of each `of said gaps.

' 6. In a transducer of the character indicated, magneticcoremeanscomprising -a plurality of pole pieces oriented to define in yaradiating surface a plurality of spaced elongated gaps, a movingconductive coil including two opposed ystretches movably supported intwo of said gaps, and piston means carried by said coil at the radiatingend thereof in clearance relation with said gaps and substantiallyfilling said gaps.

7. A transducer according to claim 6, in which said piston means has anetfective cross-section of ytrapezoidal configuration Withthe basethereof secured to said coil so as -to provide an area-.transformingelement.

8. A transducer according to claim 7, and including a 'layer ofacoustically transparent material covering the radiating surface exceptat said openings, said layer being of .a thickness to place the frontface thereof in substantially the plane of the radiating face of saidpiston.

`9. A transducer according to claim 7, and including acousticallyabsorbent supporting means lfor said coil on the non-radiating sidethereof.

l0. In a transducer of the character indicated, a baille plate ofIacoustically hard material having two relatively small laterally spacedopenings in the radiating face thereof and spaced by an vamount lessthan a wavelength at the desired response frequency, flexible diaphragmmeans peripherally secured at each of said openings .and covering each`of said openings, magnetic-circuit means establishing air gaps adjacentthe non-radiating side of said baille and symmetrically disposed withrespect to the axes of said openings, electrodynamic driving means insaid respective gaps and extending in direct force-transmitting relationwith the back side of each `of said diaphragms, a rigid housing ixedlysupporting said magnetic circuit means and defining ybetween saidmagnetic circuit means 'and a part of said housing a pressure-equalizingcavity with an external opening on a side other than the radiating sideof said transducer, `a collapsible llexible bag within said cavity andcovering said opening, Whereby fluid may enter said opening and tillsaid bag, there being a fluid-communicating opening Abetween one of saidgaps and the other side of said collapsible bag, whereby air Within saidtransducer may be maintained under ambient fluid-pressure conditions.

11. In a transducer of the character indicated, an extensivefacoustica'lly hard baffle plate, an array of separate magnet assemblies'adjacent one side of said plate, said magnet assemblies defining ineach instance Iau annular air gap immediately adjacent said bale plate,said bale plate being locally recessed adjacent each said air gap andhaving for each said recess an opening to the opposite side thereof,piston means comprising .an electrodynamic coil suspended for axialreciprocation in each of said gaps and including a part projectingthrough said 'baffle plate .at the recess therein, said projecting partcomprising a tnuncated conical area-transforming member secured at thebase to said coil, a flexible diaphragm periphenally secured to theopening in said bale plate and centnally secured to saidarea-transforming member for supporting said member with the truncatedbase substantially in the plane of said other side of said bafe plate.

References Cited in the tile of this patent UNITED STATES PATENTS

